Preterm infants, 500-1250 g birthweight will have sequential 24-hour urine samples collected at 2,4,6,8 and 10 weeks of age and evaluated for the presence of known risk factors for nephrocalcinosis. Those infants who are receiving enteral feedings at the time of the studies will be given a dose of 44Ca with a feeding to evaluate the presence of orally administered mineral in the urine. This will allow for an evaluation of the effects of care methods used in these very low birth weight infants, including medication usage and enteral feedings, on the development of urinary findings which man increase the rise of nephrocalcinosis.